Active Support, which is always taken care of by Rails engineers, includes processing to extend the Ruby default Array # sum.
Here is a summary of the behavior of #sum found when chasing the implementation of ActiveSupport in a certain library development.
[1,2,3].sum
Probably the most standard usage.
[5, 15, 10].sum
=> 3
By the way, this usage can be used without using ActiveSupport.
In the past, it seems that ActiveSupport's Array # sum operated faster than Ruby's standard Array # sum, and it was taken into the head family at a certain time, and now both are said to be the same speed.
['foo', 'bar'].sum
An error will occur without Active Support ↓
pry(main)> ['foo', 'bar'].sum
TypeError: String can't be coerced into Integer
from (pry):1:in `+'
However, with Active Support, this happens
pry(main)> ['foo', 'bar'].sum
=> "foobar"
Suddenly + turned into Array.join.
That said, it's not that you can't understand this behavior because you can concatenate strings by adding + in Ruby.
[[1,2,3], [4,5]].sum
As in the previous example, an error will occur without Active Support.
pry(main)> [[1,2,3], [4,5]].sum
TypeError: Array can't be coerced into Integer
from (pry):2:in `+'
So what happens with Active Support? It will be like this
pry(main)> [[1,2,3], [4,5]].sum
=> [1, 2, 3, 4, 5]
Perhaps some people expected it to behave the same as Array.flatten.sum?
Unfortunately the result is not sum. It will be flatten instead. why...?
What if you sum with Hash instead of Array?
pry(main)> {a: 10, b: 20}.sum
=> [:a, 10, :b, 20]
！？！？！？ Looking at the source code, I found the following description
# We can't use Refinements here because Refinements with Module which will be prepended
# doesn't work well https://bugs.ruby-lang.org/issues/13446
I see ...